Struggling actor Frankie Howerd is invited to a sprawling country home by Ray Milland to perform a dramatic reading for the Henderson family, a gathering that promises courtesy but hints at something more sinister beneath the surface. Inside, Stewart Henderson welcomes the guest with warmth, yet his earlier, discreet act of slicing a poster of Foster Twelvetrees suggests a hidden motive in a game that blends charm with menace. Early tension intensifies as Stewart’s sister Rosalie Crutchley as Jessica Henderson and their Indian servant John Bennett Patel begin rummaging through Twelvetrees’ luggage, quietly signaling that surveillance and suspicion will drive the weekend.
Twelvetrees—the titular performer—responds with an unintentionally sharp wit and a clumsy, endearing demeanor that keeps everyone off balance. After they send him to bed, Stewart and Jessica speak in coded terms about something missing from the luggage, insinuating that the missing item must be elsewhere. The plot thickens when Stewart scolds him for nearly stepping into a restricted room, explaining that his ill brother Victor lies within, a fact that raises questions about what the family is really protecting. Later, Twelvetrees is drawn into the household’s orbit as Reggie Henderson and his daughter Verity arrive, with Reggie pressing for his regular allowance and both of them reading through the family finances with a practised suspicion.
The following day brings a dingy, underground snake house that adds a chilling, symbolic layer to the intrigue. Upstairs, Twelvetrees meets Stewart’s mother, a polite-seeming figure who is kept behind a locked door. She recounts a family history of theatrics in India, but the moment she lunges for a knife, Twelvetrees is saved by Patel, who reveals that her presence is kept secret to keep her from being taken away. The encounter leaves Twelvetrees rattled, yet Stewart convinces him to perform again that evening, maintaining the illusion of a cordial family gathering while danger quietly closes in.
As Ernest Henderson and his wife Aggie join the mix, the household buzzes with financial anxieties: bounced cheques from Victor fuel their suspicions, and everyone grows intent on preventing any manipulation of Victor’s will. Verity urges Twelvetrees to investigate Victor, and the pair stumble upon a shocking sight—the bed in Victor’s room is a dummy, a ruse that confirms the family’s readiness to bend reality to protect their secrets. Stewart then reveals a second, startling truth: Twelvetrees is Victor’s secret son and thus an heir to the fortune and the house, a revelation that also hints at a potential clue to hidden diamonds on the estate. Ernest and Aggie, convinced they’ve found a key in a misquoted motto, decide to poison Twelvetrees, but Stewart foils their plan, showing that he can anticipate and outmaneuver threats within the family.
That night culminates in tragedy as Ernest is stabbed in a backroom confrontation during a Henderson family performance, forcing Twelvetrees to flee in panic. Verity pursues him, persuading him to return only after she discloses his true lineage and his rightful place in the will, making him a target not only for fortune but for the estate’s coveted diamonds. In the woods, Twelvetrees encounters Patel in another twist of fate, but the servant is mistaken for a trespasser by the Henderson matriarch and killed as he makes his way back through the grounds.
Back at the house, Twelvetrees reunites with Verity, only to discover Jessica has in her possession the dubious motto that may unlock the diamond clue, while Agnes Henderson is dead near the snake house. Twelvetrees learns that his birth name—Nigel Anthony Julian Amadeus Henderson—connects him to the estate’s long-running drama. Verity quickly recognizes that the clue lies in his initials, which form the word naja, a genus of snake, guiding him to a hidden package within the snake house. Stewart confronts him in a tense chase, a pursuit that ends with Stewart’s capture as his mother and Verity watch. In the downstairs hall, Verity levels a gun at Twelvetrees, demanding the diamonds, and he unveils the real treasure—inside the package is not merely a map but a document that, once opened, reveals the route to the estate’s diamonds. The covering paper proves to be the map, but by the time the revelation sinks in, the map is burned away in the fire.
The finale closes with a somber, orderly vigil as Stewart, Verity, and the Henderson mother are escorted away in a police cart, leaving Twelvetrees to a solitary task outside: the camera pulls away as he begins to dig in the expansive grounds, initiating a quiet, enduring search for the hidden diamonds that may forever lie just beyond reach.
